Der elektronische Grundzustand von [Fe(CO)3(NO)]−: eine spektroskopische und theoretische Studie

Iron-Catalysed Carbene Transfer to Isocyanides as a Platform for Heterocycle Synthesis

An iron-catalysed carbene transfer reaction of diazo compounds to isocyanides has been developed. The resulting ketenimines are trapped in situ with various bisnucleophiles to access a range of densely functionalized heterocycles (pyrimidinones, dihydropyrazolones, 1H-tetrazoles) in a one-pot process. The electron-rich Hieber anion ([Fe(CO)₃ NO]^- ) facilitates efficient catalytic carbene transfer from acceptor-type α-diazo carbonyl compounds to isocyanides, providing a cost-efficient and benign alternative to similar noble metal-catalysed processes. Based on DFT calculations a plausible reaction mechanism for activation of the α-diazo carbonyl carbene precursor and ketenimine formation is provided.

Spectroscopic Signature of Dynamical Instability of the Aqueous Complex in the Brown-Ring Nitrate Test

The chemistry of the brown-ring test has been investigated for nearly a century. Though recent studies have focused on solid state structure determination and measurement of spectra, mechanistic details and kinetics, the aspects of solution structure and dynamics remain unknown. We have studied structural fluctuations of the brown-ring complex in aqueous solution with ab-initio molecular dynamics simulations, from which we identified that the classically established pseudo-octahedral [Fe(H₂ O)₅ (NO)]²⁺ complex is present along with a square-pyramidal [Fe(H₂ O)₄ (NO)]²⁺ complex. Based on the inability in multi-reference calculations to reproduce the experimental UV-vis spectra in aqueous solution by inclusion of thermal fluctuations of the [Fe(H₂ O)₅ (NO)]²⁺ complex alone, we propose the existence of an equilibrium between pseudo-octahedral and square-pyramidal complexes. Despite challenges in constructing models reproducing the solid-state UV-vis spectrum, the advanced spectrum simulation tool motivates us to challenge the established picture of a sole pseudo-octahedral complex in solution.

Stable Salts of Heteroleptic Iron Carbonyl/Nitrosyl Cations

The oxidation of Fe(CO)₅ with the [NO]⁺ salt of the weakly coordinating perfluoroalkoxyaluminate anion [F‐{Al(OR^F)₃}₂]⁻ (R^F=C(CF₃)₃) leads to stable salts of the 18 valence electron (VE) species [Fe(CO)₄(NO)]⁺ and [Fe(CO)(NO)₃]⁺ with the Enemark–Feltham numbers of {FeNO}⁸ and {FeNO}¹⁰. This finally concludes the triad of heteroleptic iron carbonyl/nitrosyl complexes, since the first discovery of the anionic ([Fe(CO)₃(NO)]⁻) and neutral ([Fe(CO)₂(NO)₂]) species over 80 years ago. Both complexes were fully characterized (IR, Raman, NMR, UV/Vis, scXRD, pXRD) and are stable at room temperature under inert conditions over months and may serve as useful starting materials for further investigations.

σ-Noninnocence: Masked Phenyl-Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of -2 at a transition-metal center. For a series of formal high-valent NiIV complexes, aryl-CF3 bond-forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034-8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal NiIV centers are better described as approaching a +II oxidation state, originating from highly covalent metal-ligand bonds, a phenomenon attributable to σ-noninnocence. A direct consequence is that the elimination of aryl-CF3 products occurs in an essentially redox-neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF3 group is transferred to an electrophilic aryl group. The uncovered role of σ-noninnocence in metal-ligand bonding, and of an essentially redox-neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.

[Fe(H2 O)5 (NO)]2+ , the "Brown-Ring" Chromophore

Although the "brown-ring" ion, [Fe(H₂ O)₅ (NO)]²⁺ (1), has been a research target for more than a century, this poorly stable species had never been isolated. We now report on the synthesis of crystals of a salt of 1 which allowed us to tackle the unique bonding situation on an experimental basis. As a result of the bonding analysis, two stretched, spin-polarised π-interactions provide the Fe-NO binding-and challenge the concept of "oxidation state".

Going Homoleptic: Synthesis and Full Characterization of Stable Manganese Tetranitrosyl Cation Salts

Although similar to carbon monoxide, the chemistry of homoleptic nitrogen monoxide complexes is fundamentally unexplored compared to their carbonyl analogues. Herein we report the synthesis of the first truly homoleptic transition-metal nitrosyl cation as the salt of the weakly coordinating anions (WCAs) [Al(OR^F )₄ ]^- and [F{Al(OR^F )₃ }₂ ]^- (R^F =C(CF₃ )₃ ). These salts are easily accessible in good yields, phase pure, and were fully characterized by IR/Raman, NMR and UV/Vis spectroscopy as well as single-crystal and powder X-ray diffraction. They may serve as unprecedented simple model systems for theoretical and experimental studies of nitrosyl complexes.

The Pentagonal-Pyramidal Hexamethylbenzene Dication: Many Shades of Coordination Chemistry at Carbon

A recent report on the crystal structure of the pentagonal-pyramidal hexamethylbenzene dication C6 (CH3 )62+ by Malischewski and Seppelt [Angew. Chem. Int. Ed. 2017, 56, 368] confirmed the structural proposal made in the first report of this compound in 1973 by Hogeveen and Kwant [Tetrahedron Lett. 1973, 14, 1665]. The widespread attention that this compound quickly gained led us to reinvestigate its electronic structure. On the basis of intrinsic bond orbital analysis, effective oxidation state analysis, ring current analysis, and comparison with well-established coordination complexes, it is demonstrated that the central carbon atom behaves like a transition metal. The central (apical) carbon atom, although best described as a highly Lewis-acidic carbon atom coordinated with an anionic cyclopentadienyl ligand, is also capable of acting as an electron-pair donor to a formal CH3+ group. The different roles of coordination chemistry are discussed.

An Objective Alternative to IUPAC's Approach To Assign Oxidation States

The IUPAC has recently clarified the term oxidation state (OS), and provided algorithms for its determination based on the ionic approximation (IA) of the bonds supported by atomic electronegativities (EN). Unfortunately, there are a number of exceptions and ambiguities in IUPAC's algorithms when it comes to practical applications. Our comprehensive study reveals the critical role of the chemical environment on establishing the OS, which cannot always be properly predicted using fix atomic EN values. By identifying what we define here as subsystems of enhanced stability within the molecular system, the OS can be safely assigned in many cases without invoking exceptions. New insights about the effect of local aromaticity upon OS are revealed. Moreover, we prove that there are intrinsic limitations of the IA that cannot be overcome. In this context, the effective oxidation state (EOS) analysis arises as a robust and general scheme to derive an OS without any external guidance.

Curly arrows, electron flow, and reaction mechanisms from the perspective of the bonding evolution theory

Despite the usefulness of curly arrows in chemistry, their relationship with real electron density flows is still imprecise, and even their direct connection to quantum chemistry is still controversial. The paradigmatic description - from first principles - of the mechanistic aspects of a given chemical process is based mainly on the relative energies and geometrical changes at the stationary points of the potential energy surface along the reaction pathway; however, it is not sufficient to describe chemical systems in terms of bonding aspects. Probing the electron density distribution during a chemical reaction can provide important insights, enabling us to understand and control chemical reactions. This aim has required an extension of the relationships between the concepts of traditional chemistry and those of quantum mechanics. Bonding evolution theory (BET), which combines the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT), provides a powerful method that offers insight into the molecular mechanism of chemical rearrangements. In agreement with the laws of physical and aspects of quantum theory, BET can be considered an appropriate tool to tackle chemical reactivity with a wide range of possible applications. In this work, BET is applied to address a long-standing problem: the ability to monitor the flow of electron density. BET analysis shows a connection between quantum mechanics and bond making/forming processes. Likewise, the present approach retrieves the classical curly arrows used to describe the rearrangements of chemical bonds and provides detailed physical grounds for this type of representation. We demonstrate this procedure using the test set of prototypical examples of thermal ring apertures, and the degenerated Cope rearrangement of semibullvalene.

A Triad of Highly Reduced, Linear Iron Nitrosyl Complexes: {FeNO}(8-10)

Given the importance of Fe-NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark-Feltham numbers: {FeNO}(8) , {FeNO}(9) , and {FeNO}(10) . These {FeNO}(8-10) compounds are each low-spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}(8-10) are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark-Feltham numbers.

To Transfer or Not to Transfer? Development of a Dinitrosyl Iron Complex as a Nitroxyl Donor for the Nitroxylation of an Fe(III) -Porphyrin Center

A positive myocardial inotropic effect achieved using HNO/NO(-) , compared with NO⋅, triggered attempts to explore novel nitroxyl donors for use in clinical applications in vascular and myocardial pharmacology. To develop M-NO complexes for nitroxyl chemistry and biology, modulation of direct nitroxyl-transfer reactivity of dinitrosyl iron complexes (DNICs) is investigated in this study using a Fe(III) -porphyrin complex and proteins as a specific probe. Stable dinuclear {Fe(NO)2 }(9) DNIC [Fe(μ-(Me) Pyr)(NO)2 ]2 was discovered as a potent nitroxyl donor for nitroxylation of Fe(III) -heme centers through an associative mechanism. Beyond the efficient nitroxyl transfer, transformation of DNICs into a chemical biology probe for nitroxyl and for pharmaceutical applications demands further efforts using in vitro/in vivo studies.

HERFD-XAS and valence-to-core-XES: new tools to push the limits in research with hard X-rays?

In this perspective, the HERFD-XANES (high energy resolution fluorescence detected X-ray absorption near edge structure) and Kβ2,5- or V2C-XES (valence-to-core X-ray emission spectroscopy) methods are discussed as new and powerful tools for chemical research with hard X-rays. This includes a brief survey of the underlying physical processes and the introduction of experimental issues. The potential of both methods to overcome limitations of conventional XAS (X-ray absorption spectroscopy) and to push the limits for obtaining new information about the electronic and geometric structures of metal centers, in the solid state structure or heterogeneous catalysts, but also in metal complexes and homogeneous catalysts, is discussed by presenting a survey of representative references and recent own studies, rounded off by a conclusion and outlook.

Cooperative catalysis: electron-rich Fe-H complexes and DMAP, a successful "joint venture" for ultrafast hydrogen production

A series of defined iron-hydrogen complexes was prepared in a straightforward one-pot approach. The structure and electronic properties of such complexes were investigated by means of quantum-chemical analysis. These new complexes were then applied in the dehydrogenative silylation of methanol. The complex (dppp)(CO)(NO)FeH showed a remarkable activity with a TOF of more than 600 000 h(-1) of pure hydrogen gas within seconds.